This invention relates to a steel for a metal mold for molding glass and a mold for molding glass, which, without being chromium-plated, is suitable for use in casting high-quality glass wares with requisite excellent surface appearance, exhibits a good resistance against development of surface irregularities or thermoplastic strain in the surface under the thermal stress condition during casting operation, and has an excellent anti-heat-check life.
In general, in a high-quality metal mold for molding glass, there has frequently been used 13-Cr martensite stainless steel (equivalent to AISI 420 having a hardness of H.sub.B 235) which is subsequently chromium-plated in order to keep the mold surface from oxidation and corrosion, to improve the mold release property, ensuring a fine surface of the molded glass, and to improve the mold life.
The factors affecting the mold life include development of surface irregularities due to oxidation at high temperatures, chemical corrosion caused by Glanber's salt, formation of an irregular surface pattern due to thermal stress during the molding operation owing to insufficient mechanical strengths at high temperatures, and formation of surface irregularities of the abrasion type resulting from insufficient resistances to heat check and wear.
Accordingly, it is required for the mold material to meet the following requirements:
(1) High oxidation resistance at high temperatures.
(2) High corrosion resistance to Glauber's salt.
(3) High mechanical strengths at room temperature and at high temperatures.
(4) High heat check resistance.
In connection with the above properties, the mold material should meet the following requirements:
(5) Heat conductivity not being excessively low.
(6) Thermal expansion coefficient not exhibiting an extraordinary tendency.
(7) Good mold release property and sufficient resistance to seizure.
(8) Fine crystal grain.
(9) Susceptibility to excellent mirror finish.
In these respects, because of its insufficient resistances to oxidation and corrosion, a conventional 13-Cr martensite stainless steel cannot withstand normal usage unless it is chromium-plated. Furthermore, even if it is chromium-plated, owing to the oxidation, erosion, peeling-off and wear of the chromium-plating itself, the mold cannot acquire satisfactory durability by the present day technique particularly in the case where the mold is used in molding glass wares which are primarily required to have an excellent surface appearance.